Did the Prophet (peace be upon him) die a martyr, and is it permissible to say, "After his martyrdom, such and such happened"?

The Prophet, peace and blessings be upon him, died as a martyr due to the fever and headache resulting from the effects of the poisoned meal he ate in Khaybar, which only affected him later at the time of his appointed death, so that he might attain the status of the martyrs.